The present invention relates to industrial fluid flow measurement systems, and more particularly, to systems that provide a mass flow measurement or other fluid property measurements.
Mass flow of a fluid can be estimated using various techniques. For example, current differential pressure measurement techniques can be used as a basis for calculating mass flow. Generally, an obstruction device, such as an orifice plate, partially obstructs the fluid flow and generates a differential pressure between the upstream flow and the downstream flow. Measuring the differential pressure can provide an indication of flow. Generally, additional information is required in order for the differential pressure measurement to provide mass flow information. Specifically, information about the composition and temperature of the fluid must be known or measured. This is due, at least in part, to the fact that the differential pressure is not based only on the flow, but also on the fluid density, which itself may be a function of temperature. Further, the nature of the flow, laminar or turbulent, may affect the differential pressure reading.
Mass flow can also be calculated using a vortex flowmeter system as a basis. Vortex flowmeter systems are used in the industrial process control field for directly measuring the flow rate of a fluid through a conduit. Vortex flowmeters are typically inserted into the pipe or conduit that carries the fluid to be measured. Industry applications include petroleum, chemical, pulp and paper, mining and materials, oil and gas. The fluids to be measured are often hazardous and may be flammable or explosive. The fluids may also include condensate, deposits or other properties that can make flow measurements difficult. In these applications, safety and measurement accuracy are critical.
The operating principle of a vortex flowmeter is based on the phenomenon of vortex shedding known as the von Karman effect. As fluid passes a bluff body, it separates and generates small as eddies or vortices that are shed alternately along and behind each side of the bluff body. These vortices cause areas of fluctuating pressure that are detected by a sensor. The frequency of vortex generation is essentially proportional to fluid velocity.
Generally, vortex flowmeters provide a volumetric flow output. Quite simply, this volumetric flow output is the product of the fluid flow velocity (proportional to vortex frequency) through the conduit multiplied by the area of the conduit. Some vortex flowmeter systems are known that can provide mass flow output. Generally, such systems measure the temperature and absolute pressure of the fluid flowing from the conduit. Then, using some assumptions about the nature of the fluid and/or composition thereof, a calculation can be approximated for the mass flow of the fluid. Generally, this calculation is merely an approximation and can be susceptible to error when the composition changes. Moreover, the relative complexity of the calculations involved are difficult to provide continuously in a real-time manner.
Measuring mass flow of a fluid using both a differential pressure measurement and vortex measurement vastly simplifies the calculations and allows density and mass flow to be easily provided. Such an approach is described in United States Patent Publication US 2002/0096208 published Jul. 25, 2002. While the disclosure of that patent publication provides an effective starting point, much work remains to be done before such principles can be effectively incorporated into real-world implementations. For example, although the calculations themselves become simpler, the publication speaks of requiring two measurement devices: a differential pressure flow measuring element and a vortex flow measuring element are required. Accordingly, the real-world cost to an end user for such calculation simplicity is potentially double the equipment cost that is currently required. Systems that could provide the advantages of differential pressure and vortex measurement without the associated substantial cost increase would allow the industry to adopt such advantageous technology more readily.